Method of producing white lead



Parental Feb. 16, 1932 UNITED STATES PATENT OFFICE GUNNARD E. JOHNSON,OF EAST CHICAGO, REGINALD Cg. BOWMAN, OF GARY, AND WILLIAM J. KNOX,IR-,0]? EAMLMOND INDIANA, ASSIGNORS TO ANACONDA LEAD PRODUCTS COMPANY,OF EAST CHICAGO, INDIANA, A CORPORATION 01E DELA- WARE METHOD OFPRODUCING WHITE LEAD No Drawing.

This invention relates to improvements in the electrolytic process ofproducing white lead and is more particularly concerned with certainrefinements in the control of the process whereby we are enabled topredetermine the character or quality of White lead formed and toaccurately control, in continuous operation, the conditions which areessential to the production of the desiredproduct.

In the United States patent to Ralph M. Harrington, No. 1,308,948, it isrecognized that the character and quality of the white lead product maybe controlled by regulating the amount of carbon dioxide supplied to thecatholyte, and it is further recognized that in order to obtain rapidcarbon dioxide absorption and at the same time avoid precipitation ofwhite lead on the anode, it is necessary to maintain the catholyte ascompared with the anolyte relatively strongly alkaline.

In the operation of the-Harrington process on a commercial scale and fora period of years we have encountered certain difliculties and haveworked out certain refinements of control for overcoming suchdifiiculties, which refinements constitute the subject matter of thepresent invention.

In the first place we have found that the composition of the white leadproduced varies directly with the H00, ion or bicarbonate content of theanolyte. By virtue of this discovery we are enabled to more accuratelyand intelligently control the composition of the white lead produced andto bringabout such control in a variety of ways, i. e., by any methodwhich may serve to influence the H00 ion content of the anolyte, e g.(1) b controlling the supply of CO to the catholyte as described in theHarrington patent, (2) by maintaining the supply of CO to the catholyteconstant and adding acetic acid to the anolyte at a point outside thecell, (3) by maintaining the supply of CO, to the catholyte constant andsubjecting the anolyte to an extra electrolytic treatment to convert itssodium acetate content into bicarbonate, and (4) by maintaining thesupply of CO to the catholyte constant and supplying sodium bicarbonateas such to the anolyte at a point outside the cell. Still other methodsof con- Applieation filed January 17, 1931. Serial No. 509,534.

trolling the bicarbonate content of the anolyte might readily bedevised, the foregoing being suggested merely as examples.

In the second place we have found that the carbonate salt content of.the anolyte must be kept 1owmuch lower than is required in order toavoid formation of white lead incrustations on the anodein order topermit washing the white lead free of these salts in the finishing ofthe white lead separated from v i the anolyte. This is important in thecommercial production of white lead by the electrolytic method, sincethe presence of any considerable amount of alkali metal carbonate in thewhite lead product renders it unfit for use as paint pigment. We havefound that the difliculty in washing the white lead sufliciently free ofadsorbed salts increases with increasing concentration of earbonate saltin the anolyte to a point at which it is practically impossible toeliminate them. This may be explained by adsorption of the salts in thewhite lead particles, but it seems more probable that a reaction mayoccur as follows :v

inert cathode suspended in the catholyte cony taining sodium carbonateand separated from the anolyte by a suitable diaphragm, and to which COis supplied, all as disclosed in the above mentioned Harrington patent,the anolyte solution is sampled and analyzed at regular intervals, sayevery hour, and its" bicarbonate content adjusted to and maintainedatthe desired figure by any suitable method such as one of those referredto above.

For example, in a cell operated at a cur rent density of 30 amperes persquare foot of cathode area and producing approximately .011 pounds ofwhite lead perampere hour and operating at an anolyte displacementtained at not exceeding about twenty-five one-hundredths of one percent,(0.25%) and preferably in the neighborhood of six onehundredths of onepercent (0.06%). The significance of the limited carbonate salt contentof the anolyte specified may be appreciated by comparison with thelimitation of the carbonate salt content of the anolyte necessary toavoid troublesome precipitation of white lead on the anode in accordancewith the Harrington patent. In this case the carbonate salt content ofthe anolyte may run as high as six tenths-of one percent (0.60%), whichcontent, however, will produce White lead containing so. much carbonatesalt that it is not feasible in commercial operation to wash itsulhciently free of such salt as to produce a first grade white leadpaint pigment.

As ha been stated, the two principal points of refin ment of the processdescribed in the Harrington patent constituting the present inventionare (1) limiting the carbonate salt content of the anolyte to apredetermined maximum to permit the substantially complete removal ofsuch salt from the precipitated white lead and the production of a whitelead pigment suitable for use as a paint pigment, and (2) control of thecomposition of the White lead by reference to the bicarbonate content ofthe anolyte. In explanation of the last named refinement it may be saidthat the white lead formation is reprev sented by the following reactionequation:

3Pb 02151302 2 +4Na CO 2H2O 2PbCO .Pb-(OH) 2 +2NaHCO snaozn o Thecomposition of the basic lead salt pre-' crease until finally only PbCQwould be produced. In other words the, CO content of the precipitatedwhite lead bears a direct relation to the NaHCU concentration of theanolyte. This has been found to be true in actual operation. In order tocontrol the composition of the white lead produced therefore,

- it is necessary to control the NaI-IC O content of the anolyte, e. g.by controlling the ratio of OH to HCO ions transferred to the anolytefrom the catholyte through the diapable under electrolysis of yielding alead sol vent and containing bicarbonatesalt the steps which consist indetermining the bicarbonate concentration in the anolyte correspondingto the precipitation of white lead of the desired composition, andmaintaining said bicarbonate content in the anolyte by maintaining auniform and deficient carbonation of the catholyte and adding an acid tothe anolyte.

2. In the process of producing white lead by electrolysis in a bifiuidcell in which the White lead is precipitated in an anolyte capable underelectrolysis of yielding a lead solvent and containing bicarbonate saltthe steps which consist in determining the bicarbonate concentration inthe anolyte corresponding tothe precipitation of white lead of thedesired composition, and maintaining said bicarbonate content intheanolyte by maintaining a uniform and deficient carbonation of thecatholyte and adding a bicarbonate to the anolyte.

In testimony whereof, We afiix our signatures.

' GUNNARD E. JoHNs'oN. REGINALD G. BOWMAN. WILLIAM J. KNOX, JR.

